fly ash is one of the residues generated in combustion. It comprises fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. Fly ash material can solidify while suspended in the exhaust gases and is collected by electrostatic precipitators Of filter bags. Since the particles solidify while suspended in the exhaust gases, fly ash particles can be generally spherical in shape and range in size from 0.5 μm to 100 μm. Fly ash can comprise silicon dioxide (SiO2) (which can be present in two forms: amorphous, which is rounded and smooth, and crystalline, which is sharp, pointed and hazardous); aluminium oxide (Al2O3 ) and iron oxide (Fe2O3), Fly ashes can also comprise calcium oxide (CaO), Fly ashes can also be highly heterogeneous. They can comprise a mixture of glassy particles with various identifiable components.
For example, fly ash can refer to ash produced during combustion of coal. Depending upon the source and makeup of the coal being burned, the components of fly ash vary considerably,
In the past, fly ash was generally released into the atmosphere, but pollution control equipment mandated in recent decades now require that it be captured prior to release. Fly ash is generally captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys of coal-fired power plants, and together with bottom ash removed from the bottom of the furnace is in this case jointly known as coal ash. For example, in the US, fly ash can be generally stored at coal power plants or placed in landfills. About 43 percent is recycled,[3] often used to supplement Portland cement in concrete production. However, some scientists have expressed health concerns about this.
In the past, fly ash produced from coal combustion was simply entrained in flue gases and dispersed into the atmosphere. This created environmental and health concerns that prompted laws which have reduced fly ash emissions to less than 1 percent of ash produced. Worldwide, more than 65% of fly ash produced from coal power stations is disposed of in landfills and ash ponds. The recycling of fly ash has become an increasing concern in recent years due to increasing landfill costs and current interest in sustainable development. As of 2005, U.S. coal-fired power plants reported producing 71.1 million tons of fly ash, of which 29.1 million tons were reused in various applications. If the nearly 42 million tons of unused fly ash had been recycled, it would have reduced the need for approximately 27,500 acre·ft (33,900,000 m3) of landfill space.
There is thus a need for at least an alternative process for treating, recycling and/or valorizing fly ash.